There are few happy consequences of the emergence of the Islamic State of Iraq and the Levant, which has swept across Iraq and Syria since the beginning of the year. A rare shaft of light, however, is to be found in the rapprochement between the two biggest branches of the Islamic mainstream.

Shia and Sunni Islam have been at odds for most of the past decade. But the self-declared caliphate of terror known as Isis presents them with a common challenge. And they are beginning to put aside their differences in an effort to meet it.

The grand clerics of both schools – the Shia from Tehran, Qom and Najaf, and the Sunni from Riyadh and Cairo – have called upon their followers to take up arms against Isis. Political leaders have also declared their support for an intensified campaign. When the Iranian and Saudi foreign ministers met in New York in September, both diplomats hailed the opening of a new chapter in relations between their countries.